#!/usr/bin/env perl 
use strict;
use warnings;

my @files = (
'000001.OrderID.html',
'000002.OrderID.cgi',
'000004.OrderID.js',
'000005.OrderID.txt',
'000006.OrderID.cgi',
'OrderID.master'
);

my $i = 1;
for my $infile (sort grep /^\d{6}\./, @files) {
    my $repl = sprintf ("%6.6i", $i++);
    (my $outfile = $infile) =~ s/^\d{6}/$repl/;
    rename $infile, $outfile;
}